1. Field of the Invention
The invention relates to a polymer, producing method thereof, and photorefractive composition. More particularly, the invention relates to polymers and copolymers that contain fullerene moiety at the backbone (co)polymer chain, and to methods of making such polymers. Also, the invention relates to the compositions that include such polymer and provide photorefractive capabilities.
2. Description of the Related Art
Photorefractivity is a phenomenon in which the refractive index of a material can be altered by changing the electric field within the material, such as by intense laser beam irradiation. The change of refractive index is achieved by a series of steps, including: (1) charge generation by laser irradiation, (2) charge transport, resulting in separation of positive and negative charges, and (3) trapping of one type of charge (charge delocalization), (4) formation of a non-uniform internal electric field (space-charge field) as a result of charge delocalization, and (5) refractive index change induced by the non-uniform electric field.
Therefore, good photorefractive properties can be seen only for materials that combine good charge generation, good charge transport, or photoconductivity, and good electro-optical activity.
Photorefractive materials have many promising applications, such as high-density optical data storage, dynamic holography, optical image processing, phase conjugated mirrors, optical computing, parallel optical logic, and pattern recognition.
Originally, the photorefractive effect was found in a variety of inorganic electro-optical (EO) crystals, such as LiNbO3. In these materials, the mechanism of the refractive index modulation by the internal space-charge field is based on a linear electro-optical effect.
In 1990 and 1991, the first organic photorefractive crystal and polymeric photorefractive materials were discovered and reported. Such materials are disclosed, for example, in U.S. Pat. No. 5,064,264, to Ducharme et al. Organic photorefractive materials offer many advantages over the original inorganic photorefractive crystals, such as large optical nonlinearities, low dielectric constants, low cost, lightweight, structural flexibility, and ease of device fabrication. Other important characteristics that may be desirable depending on the application include sufficiently long shelf life, optical quality, and thermal stability. These kinds of active organic polymers are emerging as key materials for advanced information and telecommunication technology.
In recent years, efforts have been made to optimize the properties of organic, and particularly polymeric, photorefractive materials. As mentioned above, good photorefractive properties depend upon good charge generation, good charge transport, also known as photoconductivity, and good electro-optical activity. Various studies that examine the selection and combination of the components that give rise to each of these features have been done.
The photoconductive or charge transport capability is frequently provided by incorporating materials containing phenyl amine derivative groups. Some examples of phenyl amine derivative groups are carbazole, triphenyl amine, or tetraphenyldiamine group containing derivatives.
Typical examples of carbazole, triphenyl amine, or tetraphenyldiamine group containing derivatives are carbazoyl alkyl derivative, carbazoyl type polymer, polyvinylcarbazole (PVK), triphenyl amine alkyl derivative, triphenyl amine type polymer, and tetraphenyldiamine (TPD) group containing polymers.
The electro-optical capability is generally provided by including chromophore or dye compounds, such as an azo-type or other electron donor and acceptor functional group containing derivatives. The charge generation capability can be generally obtained by a material known as a sensitizer, including wide range of fullerene derivatives, which can generate photo-electron by light irradiation.
Usually, fullerene derivative compounds provide better photo-electron generation ability than other fluorenone derivatives, which also work as a good photo-electron generation sensitizer.
The fullerenes are general novel class materials which are composed of only carbon atom and have ball shape chemical structure. Typically, C60 is known as a prototype. As other examples, C70, C76, C78, C84 and their mixture are also categolized as fullerenes. Furthermore, chemically modified derivatives are also belong to a class material of fullerene. The soccer-ball-shaped molecules possess three-dimensional p-delocalized electrons, a property that gives rise to a large nonresonant, instantaneous response.
The photorefractive composition may be made simply by mixing these molecular components that provide the individual properties required into a host polymer matrix. Several composition which showed good photorefractivity have been developed and studied.
For example, in PVK-based materials, the space-charge field that gives rise to the change in refractive index is built up on a sub-second time scale because of the high charge transport ability of the PVK matrix.
Japanese Patent Application Laid-open JP-A 1998-333195, to Showa Denko, discloses acrylate-based polymers incorporating triphenylamine groups as charge transport agents. Fast response times (50 msec. at 70 V/μm biased voltage), although there is no description or data regarding diffraction efficiency.
Also, there are other approach to put the photoconductivity (charge transport) capability part and the non-linear optical capability into one single polymer chain. It has been recognized that it would be desirable to prepare bi-functionalised photorefractive polymers, that is, polymers in which both the photoconductivity and the non-linear optical capability reside within the polymer itself.
As examples of these type polymers, PVK polymers in which some of the carbazole groups are tricyanovinylated have been made (N. Peyghambarian et al., Applied Phys. Lett., 1992, 60, 1803). Subsequently, the same group has reported PVK-based materials with an fast response time and a very high photoconductivity. (N. Peyghambarian et al., J. Mater. Chem., 1999, 9, 2251).
A number of efforts at materials improvement have used methacrylate-based polymers and copolymers that include photoconductive and chromophore side groups. A paper by T. Kawakami and N. Sonoda, (Applied Phys. Lett., 1993, 62, 2167.) discloses acrylate-based polymers containing dicyanovinylideneyl phenylamines as charge transport groups.
A report by H. Sato et al., (Technical report of IEICE., 1995, OME-95-53, OPE95-94, 43) describes the preparation of several copolymers having both charge transport components and non-linear optical components in the side groups of the copolymer. However, the charge transport speeds seem to be too slow for good photorefractive materials.
A paper by Van Steenwickel et al. (Macromolecules, 2000, 33, 4074) describes acrylate-based polymers that include carbazole-based side chains and several stilbene-type side chains. The paper cites a high diffraction efficiency of 60% at 58 V/μm, but a slow response time of the sub-second order.
A paper by Y. Chen et al. (Modern Optics, 1999, 46, 1003) discusses a methacrylate polymer that has both carbazole-type side chains to provide charge transport capability and nitrophenyl azo-type side chains to provide non-linear optical capability. The materials again show slow response times of over 20 sec.
All of the materials described above utilize low molecular weight sensitizer molecule as an additive. Particularly, fullerene derivatives are mostly used for a sensitizer, because fullerene gives the most efficient photo-electron generation. However, fullerene derivatives have very low solubility with either solvents or other components. Sometimes the fullerenes are clustered out into small solid particle in the photorefractive composition, due to the small solubility into components. This clustering phenomenon make compositions less transparent composition or light scattering, which leads to poor photorefractivity. Furthermore, the small solid particle can cause electric breakdown, when high voltage is applied onto the photorefrative composition during sample measurement. In order to avoid this kind of problem, new type of the fullerene incorporation methods have been demanded.
In recent years, a new type of polymerization, termed living radical polymerization, has been developed for polymerization of functional monomers, including methacrylate and styrene derivatives. Living radical polymerization differs from conventional radical polymerization in that the polymer growth terminals can be temporarily protected by protection bonding. This enables polymerization to be well controlled, including being stopped and started at will.
This process can be used to prepare homopolymers and copolymers, including block copolymers. Details of the living radical polymerization method are described in the literature. They may be found, for example, in the following papers:    1. T. Patten et al., “Radical polymerization yielding polymers with Mw/Mn˜1.05 by homogeneous atom transfer radical polymerization”, Polymer Preprints, 1996, 37, 575.    2. Matyjasewski et al., “Controlled/living radical polymerization. Halogen atom transfer radical polymerization promoted by a Cu(I)/Cu(II) redox process”,Macromolecules, 1995, 28, 7901.    3. M. Sawamoto et al., “Ruthenium-mediated living radical polymerization of methyl methacrylate”, Macromolecules, 1996, 29, 1070.
Living radical polymerization is also described in U.S. Pat. No. 5,807,937 to Carnegie-Mellon University, which is incorporated herein by reference in its entirety.
The only example known to the present inventor of fullerene-containing polymer preparation by living radical polymerization is in a paper by F. M. Li et.al. (Macromolecules, 2000, 33, 1948). This reference discloses the polymerization for a C60 fullerene-containing styrene polymer, using a copper halide catalysis. No photorefractive or electro-optical performance data are reported in the citation.